The present invention is directed to aqueous processable, photopolymerizable compositions having superior flexibility while preserving important properties for their use as permanent coatings for the protection of printed circuitry.
Photopolymerizable resist materials are known, for example, from U.S. Pat. Nos. 3,469,982 and 3,547,730, which describe a film resist with a sandwich structure in the form of a photopolymerizable layer between a cover sheet and a temporary support. This film resist can, for instance, be laminated on a copper base, exposed imagewise and developed with organic solvents or aqueous solutions, whereby a defined resist layer is formed. Typically the copper base for a printed circuit board is rigid, with limited flexibility of just a few degrees such as the conventional copper-clad fiber-glass epoxy laminates. More recently, printed circuits are being prepared on highly flexible film substrates to form electronic packages which may be folded and refolded one or more times to fit a specified configuration or a dynamic mechanical operation.
The defined resist layer thus produced can now be selectively etched, electroplated or treated with solder on the substrate. Particulary high demands are placed on photoresist films if they are used as permanent coatings that function as solder resists or masks. In this case, the developed, photopolymerized layer must withstand temperatures up to 300.degree. C. without degradation, loss of adhesion or accumulation of residues contained in or on the molten solder. With the advanced technology of today's primed circuit boards, it is important to have the capability to photoimage a solder mask. According to the current state of the art, such solder masks can be made by spraying, coating or calendering liquid compositions on a substrate or also by laminating a dry film on a substrate.
Due to the adverse environmental impact of solvents, aqueous developable photopolymer systems with fast development are now preferred. The use of photopolymer resists with acid functions, primarily carboxyl functions, is known to impart aqueous processability. However, these groups are disadvantageous in many subsequent steps or events. In the case of photoresists, delamination of the resist is observed in alkali etching or gold plating, in the case of solder masks, inadequate climatic resistance results. Modification of the carboxyl groups with melamine compounds to overcome the named disadvantages is known (EP 0 115 354 and U.S. Pat. No. 4,247,621).
The utilization of polymers containing carboxylic acid groups that are subsequently converted to less reactive and less moisture sentitive species is also known. U.S. Pat. No. 4,987,054 discloses a photopolymerizable formulation yielding improved properties containing an acid copolymeric binder wherein a copolymer structural unit is the half acid/amide of a dicarboxylic acid. The disclosed formulations are used with conventional rigid printed circuit boards, processed with wholly aqueous alkaline solutions and are storage stable. European Patent Application 0 430 175 discloses a photopolymeric system similar to U.S. Pat. No. 4,987,054.
As the electronic industry is driven towards faster, more reliable and compact devices, there is an increasing need in the printed circuit field for a more pliable, permanent coating that can withstand typical manufacturing process conditions, such as molten solder, and continuously varying environmental conditions while maintaining its integrity. The capability of such a coating to stand up to varied conditions and, in addition, be photoimageable and aqueous processable would constitute an advancement of the art. Current protective coatings for polyimide flexible circuitry require mechanical punching or drilling before lamination in an overall costly, low productivity process. Of particular use would be a photoimageable, aqueous processable, permanent coating for use with flexible circuitry that can be made with conventional lower cost photoforming manufacturing processes that have higher resolution capability, wherein the flexible circuits can be subjected to flexural stress and maintain functionality.